1. Field of the Invention
The present invention relates to a press molding glass material usable for obtaining a glass optical element by precision mold pressing and a manufacturing method thereof, and relates to a manufacturing method of an optical element using the abovementioned press molding glass material.
2. Description of the Related Art
As a method of manufacturing an optical element such as a glass lens or the like, there has been known a method of precisely press molding a press molding glass material (hereinafter, referred to also as “glass material” or “glass preform”) by using an upper die and a lower die which have molding surfaces facing each other (referred to as “precision press molding method”, “precision mold press method” or the like). This method is a method of obtaining a glass optical element by injecting optical glass solidified from a melt state to a predetermined shape or optical glass polishing-processed to a predetermined shape into the inside of a molding die and by thermally press-molding the optical glass.
In the precision press molding method, it is possible to form an optical functional surface by transcribing a precisely-processed molding surface of a molding die to a glass material, so that it is not necessary to carry out a machine process such as an optical grinding or the like for the construction of the optical functional surface after the press molding. Consequently, with the use of precision press molding method, it is possible to provide a high-performance lens inexpensively. However, in the precision press molding method, the press molding glass material and the molding surface of the molding die contact closely under a high-temperature state, so that an aspect in which a fusion occurs when a chemical reaction occurs at the interfacial surfaces thereof and the mold release property deteriorates becomes a problem.
For the procedure of improving the mold release property at the time of press molding, there has been proposed a procedure of covering the surface of the press molding glass material by an oxide film such as shown, for example, in Patent Document 1: Japanese examined patent publication H2-1779, Patent Document 2: Japanese unexamined patent publication H7-118025, Patent Document 3: Japanese unexamined patent publication H8-198631 and Patent Document 4: Japanese unexamined patent publication No. 2011-1259 or English language family member US2012/135199A1. The contents of the above applications are expressly incorporated herein by reference in their entirety.
In the Patent Documents 1 to 4, there is disclosed a silicon oxide film as an oxide film covering the surface of the press molding glass material, but according to the investigation of the present inventors, it was still not possible for the conventional silicon oxide film disclosed in the Patent Documents 1 to 4 to adequately suppress the fusion between the glass molding material for press molding and the die.
On the other hand, with regard to a press molding glass material including a silicon oxide film on the surface thereof, Japanese unexamined patent publication No. 2011-136870, published on Jul. 14, 2011, which is expressly incorporated herein by reference in its entirety, describes that there can be seen good correlation between the surface free energy measured by a three-solution method and the yield in press molding and that it is possible to improve the yield in press molding depending on a material whose surface free energy measured by a three-solution method is equal to or less than 75 mJ/m2 within glass materials for press molding including silicon oxide films on the surfaces thereof, the thickness of which is less than 15 nm.
In the press molding glass material described in the previous application mentioned above, the film thickness of the abovementioned silicon oxide film is defined to be less than 15 nm in order to suppress the fusion with respect to the molding die. However, as a result of an investigation of the present inventors, it became clear, in order to obtain a molded glass body with a desired shape, that in a case in which it is necessary to deform the press molding glass material largely in press molding, there sometimes exists a case, in the press molding glass material described in the abovementioned previous application, in which it is difficult to obtain the molded glass body with a desired shape.